Option to add class to in Redactor editor

I am using the Redactor editor as part of Perch. I would like to give the editor the ability to (optionally) add the class of "button" to any <a> tag they add. This could be via an additional option in the existing Add a Link modal, or could be a separate button in the editor toolbar.

Does anyone have any guidance as to the best way to achieve this? Any pointers appreciated.

This is my current config setup:

config.plugins = [];
config.buttons = ['format','bold','italic','deleted','link','lists'];
config.formatting = ['p', 'blockquote', 'h2', 'h3', 'h4'];

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